Inorganic light-emitting diodes (LEDs) are widely used in electronic devices as indicators and in displays. LEDs include a multi-layer doped crystalline semiconductor structure that responds to electrical current by emitting light. The frequency of the light emitted depends upon the bandgap and doping of the semiconductor. Electrical current is provided by a power supply connected to electrical contacts formed on the semiconductor.
Semiconductor materials have a large index of refraction, for example 3.96 compared to essentially 1.0 for air. Much of the light emitted from an LED is therefore trapped due to total internal reflection and is eventually converted to heat. This phenomenon is one of the greatest causes of LED inefficiency. To extract trapped light from an LED, a variety of light extraction and diffusion structures in the LED are proposed and are the subject of much research and development. For example, a hemispherical surface is often used to reduce total internal reflections in LEDs. U.S. Pat. No. 8,847,274 describes an LED encapsulated in a transparent metal oxide dome as well as a reflector. U.S. Pat. No. 8,680,556 also illustrates a dome structure with a high-reflectivity layer arranged to reflect emitted light. A light extraction layer is also shown together with a lateral-geometry LED having electrical contacts on a common side and a contact mesa.
Inorganic light-emitting diode displays can be formed using micro-LEDs (for example having an area less than 100 microns square or having an area small enough that it is not visible to an unaided observer of the display at a designed viewing distance) are known. However, micro-LED structures useful for enabling such transfer printing processes are not necessarily compatible with known light-extraction structures.
There remains a need, therefore, for improvements in light-emitting diode structures and manufacturing methods that are compatible with micro transfer printing and are effective in extracting light from a light-emitting diode.